Since the introduction of the "AD Member" mode it is not possible to edit mailAlternativeAddress via UMC or udm-cli when the object is synced from Active Directory.
This is caused by the setting "readonly_when_synced=1" for mailAlternativeAddress in /usr/share/pyshared/univention/admin/handlers/users/user.py (package python-univention-directory-manager).
As there seems to be no mechanism where any of the AD-attributes is mapped to mailAlternativeAddress, there is no reason to lock the attribute for a synced object.